US20090000585A1

US20090000585A1 - Camshaft Support Structure of an Internal Combustion Engine and Assembly Method Thereof

Info

Publication number: US20090000585A1
Application number: US12/087,650
Authority: US
Inventors: Takanori Sasaki; Tetsuhi Suzuki; Atsunori Kumagai; Shoichi Tateyama
Original assignee: Toyota Motor Corp
Current assignee: Toyota Motor Corp
Priority date: 2006-01-19
Filing date: 2007-01-18
Publication date: 2009-01-01
Also published as: EP1974127B1; DE602007010094D1; CN101371010A; EP1974127A2; JP2007192105A; WO2007083224A2; KR101051244B1; KR20080079686A; RU2008129673A; WO2007083224A3; RU2393360C2; JP4144625B2; CN101371010B

Abstract

A lash adjuster support portion is integrally formed with a lower cam carrier. The lower cam carrier is fixed onto a cylinder head into which an intake valve and an exhaust valve have been fitted. After a lash adjuster is set in the lash adjuster support portion, a rocker arm is placed on the intake side and a rocker arm is placed on the exhaust side. Camshafts are then placed on the lower cam carrier such that the rocker arm contacts an intake cam and the rocker arm contacts an exhaust cam. An upper cam carrier is then assembled to the lower cam carrier, thus completing the camshaft support structure.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention
The invention relates to a camshaft support structure of an internal combustion engine and an assembly method of that camshaft support structure. More particularly, the invention relates to a structure for supporting a camshaft in an internal combustion engine having a valve mechanism that includes a lash adjuster, and an assembly method of that structure.
2. Description of the Related Art
Japanese Patent Application Publication No. JP-A-2002-38909, for example, describes a known internal combustion engine having a valve mechanism that includes a lash adjuster. This valve mechanism also has a rocker arm which is a member for transmitting pressing force from a cam to an intake valve or an exhaust valve. One end of this rocker arm abuts against a valve stem of the intake valve or exhaust valve.
The other end of the rocker arm is supported by the lash adjuster. Also, the cam abuts against the rocker arm from the side opposite the lash adjuster and the valve stem. That is, the rocker arm is sandwiched between the lash adjuster and the valve stem which are positioned at both ends on one side of the rocker arm, and the cam which is positioned in substantially the center on the other side of the rocker arm.
In this state, when the cam pivots and the cam nose starts to abut against the rocker arm, the rocker arm starts to pivot about the support portion formed by the lash adjuster. As a result, the valve stem lifts in the direction that opens the valve so that the intake valve or exhaust valve opens. Thereafter, when the cam nose passes through the point where it abuts against the rocker arm, the lift amount of the valve stem decreases and the intake valve or exhaust valve closes again.
In order to keep the motion of the valve mechanism smooth, the rocker arm and the cam must be kept in close contact. On the other hand, changes over time such as wear and the like occur in members such as the cam and the rocker arm. The lash adjuster is a member provided to absorb the effects of these kinds of changes over time so that a predetermined pressing force is constantly applied to the rocker arm. That is, if the lash adjuster constantly applies a predetermined pressing force to the rocker arm, then the rocker arm and the cam can be kept in close contact regardless of changes over time such as wear and the like. Therefore, stable operation of the valve mechanism is able to be maintained according to the foregoing related internal combustion engine.
The support portion of the lash adjuster is typically formed as part of the cylinder head. In contrast, the support portion in the related internal combustion engine described above is formed integrated with the head cover. This structure enables the structure of the cylinder head to be simplified, which has is beneficial in many ways. For example, it improves productivity and also makes the internal combustion engine lighter. In this respect, the related structure is superior to the typical structure.
Assembly of the valve mechanism described above requires the following steps: 1) assembling the intake valve and exhaust valve and the like to the cylinder head, 2) assembling the lash adjuster in the support portion, 3) placing the rocker arm on the valve stem and lash adjuster, 4) assembling the camshaft from above the rocker arm, and 5) assembling the head cover to the cylinder head.
However, with the related structure described above, the support portion of the lash adjuster is fixed to the head cover. Therefore, after the lash adjuster has been assembled in the support portion, it is extremely difficult to place the rocker arm on the valve stem and lash adjuster. Therefore, although the related structure described above is beneficial in that it enables the cylinder head to be simplified, in reality the structure is difficult to assemble.

SUMMARY OF THE INVENTION

This invention provides a camshaft support structure of an internal combustion engine, and an assembly method thereof, which simplifies the structure of the cylinder head and which can easily be assembled in a production line.
A first aspect of the invention relates to a camshaft support structure of an internal combustion engine, which includes a cylinder head, a lower cam carrier that supports a camshaft from the cylinder head side, an upper cam carrier that supports the camshaft from the side opposite the lower cam carrier, and a lash adjuster support portion that supports a lash adjuster that supports a rocker arm that abuts against a cam. The lash adjuster support portion is provided separately from the cylinder head and connected to the lower cam carrier.
According to this first aspect, the lash adjuster support portion is provided separately from the cylinder head, thereby simplifying the structure of the cylinder head. Also according to the first aspect, the valve mechanism can be assembled according to the following steps: 1) fitting an intake valve or an exhaust valve into the cylinder head, 2) placing the lash adjuster support portion and the lower cam carrier on the cylinder head, 3) placing the rocker arm on a valve stem of the intake valve or exhaust valve and the lash adjuster that has been assembled in the lash adjuster support portion, 4) placing the camshaft on the lower cam carrier so that the rocker arm and the cam overlap, and 5) assembling the upper cam carrier to the lower cam carrier, thereby completing the support structure of the camshaft. In this way, the first aspect realizes a camshaft support structure that can be easily assembled in a production line while simplifying the structure of the cylinder head.
Also, according to a second aspect of the invention, in the first aspect, the internal combustion engine is provided with a plurality of the lower cam carriers, and the plurality of lower cam carriers are connected together by a frame to form a ladder frame type lower cam carrier.
According to the second aspect, the lash adjuster support portion can be connected to the ladder frame type lower cam carrier. This ladder frame type lower cam carrier is highly rigid so according to the second aspect, the lash adjuster support portion can be rigidity supported so the amount of wobble of the lash adjuster can be kept sufficiently small.
Also, according to a third aspect of the invention, in the first or second aspect, the lash adjuster support portion is integrally formed with the lower cam carrier.
According to the third aspect, the lash adjuster support portion is integrally formed with the lower cam carrier. Therefore, this third aspect makes it possible to form the lash adjuster support portion and the lower cam carrier simultaneously, as well as obtain a sufficiently strong connection between them.
Also, according to a fourth aspect of the invention, in any one of the first to the third aspects, the lash adjuster support portion is made of material that is lighter than aluminum.
According to the fourth aspect, the lash adjuster support portion can be made of material that is lighter than aluminum. Making the lash adjuster support portion out of lightweight material in this way enables the weight of the upper portion of the internal combustion engine to be reduced. Therefore, the fourth aspect simultaneously reduces the weight of the internal combustion engine and lowers its center of gravity.
Also, according to a fifth aspect of the invention, in any one of the first to the fourth aspects, the internal combustion engine has a plurality of cylinders arranged inline, a plurality of the lash adjusters are provided corresponding to the plurality of cylinders, and the lash adjuster support portion extends so as to support all of the lash adjusters and is provided with a first oil passage for supplying oil to the lash adjusters.
According to the fifth aspect, a first oil passage for supplying oil to the lash adjusters is provided in the lash adjuster support portion. Therefore, this fifth aspect simplifies the structure for supplying oil to the lash adjuster of each cylinder.
Also, according to a sixth aspect of the invention, in the fifth aspect, the internal combustion engine is provided with an intake valve and an exhaust valve for each cylinder, and the lash adjuster support portion is arranged in between the intake valve and the exhaust valve.
According to the sixth aspect, the lash adjuster support portion can be arranged between the intake valve and the exhaust valve. Therefore, a single lash adjuster support portion can support both the lash adjusters for the exhaust valves and the lash adjusters for the intake valves. This structure also makes it possible to make the distances from the first oil passage in the lash adjuster support portion to the lash adjusters on both the intake side and the exhaust side the same. Therefore, the sixth aspect simplifies the structure for supplying oil under the same conditions to the lash adjusters on the intake side and the exhaust side.
Also, according to a seventh aspect of the invention, in the fifth or sixth aspect, the camshaft support structure of an internal combustion engine also includes a second oil passage for supplying oil to a bearing portion formed by the lower cam carrier and the upper cam carrier, and the second oil passage is formed in a connecting body that includes the lash adjuster support portion, the lower cam carrier, and the upper cam carrier, and communicated with the first oil passage.
According to the seventh aspect, both the first oil passage for supplying oil to the lash adjusters and the second oil passage for supplying oil to the bearing portion can be formed in the connecting body that includes the lash adjuster support portion, the lower cam carrier, and the upper cam carrier. That is, according to the seventh aspect, all of the structures to supply the necessary oil in the support structure of the camshaft can be provided elsewhere than on the cylinder head. Accordingly, the seventh aspect can sufficiently simplify the structure of the cylinder head.
Also, an eighth aspect of the invention relates to an assembly method of a camshaft support structure of an internal combustion engine. This assembly method includes the steps of fixing a lower cam carrier having a lash adjuster support portion onto a cylinder head into which an intake valve and an exhaust valve have been fitted; setting a lash adjuster in the lash adjuster support portion; placing a rocker arm on the intake side and a rocker arm on the exhaust side; placing camshafts on the lower cam carrier so that the rocker arm on the intake side contacts an intake cam and the rocker arm on the exhaust side contacts an exhaust cam; and assembling an upper cam carrier to the lower cam carrier.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and further objects, features and advantages of the invention will become apparent from the following description of preferred embodiments with reference to the accompanying drawings, wherein like numerals are used to represent like elements and wherein:

FIG. 1 is a sectional view of an internal combustion engine having a camshaft support structure according to an example embodiment of the invention; and

FIG. 2 is a perspective view of a ladder frame type lower cam carrier and a lash adjuster support portion of the camshaft support structure shown in FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Structure of the Example Embodiment

FIG. 1 is a sectional view of a structure according to an example embodiment of the invention. As shown in the drawing, the structure of the embodiment has an internal combustion engine 10. This internal combustion engine 10 is a multiple cylinder type engine having a plurality of cylinders arranged inline. FIG. 1 shows the structure with the internal combustion engine 10 cut along a plane that passes through one of the cylinders.
The internal combustion engine 10 includes a cylinder head 12 which has an intake port 14 and an exhaust port 16. A cylinder top inside surface 18 that partially defines a space in each of the cylinders is also formed on the cylinder head 12. The intake port 14 and the exhaust port 16 both open to the cylinder top inside surface 18.
An intake valve 20 which opens and closes the cylinder top inside surface 18 is provided in the intake port 14. Similarly, an exhaust valve 22 which also opens and closes the cylinder top inside surface 18 is provided in the exhaust port 16. The intake valve 20 has a valve stem 24 which is slidably retained by a valve guide 28, and the exhaust valve 22 has a valve stem 26 which is slidably retained by a valve guide 30.
Retainers 32 and 34 are fixed near the upper ends of the valve stems 24 and 26, and valve springs, not shown, are arranged around the valve stems 24 and 26. These valve springs urge the retainers 32 and 34 upward in the drawing, i.e., in the closing direction of the intake valve 20 and the exhaust valve 22.
A lash adjuster support portion 40 is provided between the valve stem 24 of the intake valve 20 and the valve stem 26 of the exhaust valve 22. The lash adjuster support portion 40 is integrally formed with a ladder frame type lower cam carrier 42. Upper cam carriers 44 are fixed to the ladder frame type lower cam carrier 42 from above by fastening bolts 45.
FIG. 2 is a perspective view of a connecting body 46 that includes the lash adjuster support portion 40, the ladder frame type lower cam carrier 42, and the upper cam carriers 44. The cylinder head 12 of the internal combustion engine 10 is made of aluminum or cast iron. In contrast, the connecting body 46 is made of magnesium.
As shown in FIG. 2, the ladder frame type lower cam carrier 42 has an outer frame portion 48 and a plurality of lower cam carriers 50. The outer frame portion 48 is a portion that is fixed to the peripheral edge portion of the cylinder head 12. The lower cam carriers 50 are members that are arranged between the cylinders of the internal combustion engine 10 and are connected together by the outer frame portion 48.
Lower bearing portions 52 for supporting an intake camshaft and an exhaust camshaft, neither of which is shown, are provided on the lower cam carriers 50. Upper bearing portions 54 that form pairs with the lower bearing portions 52 are provided on the upper cam carriers 44. The intake camshaft and the exhaust camshaft are rotatably retained by the lower bearing portions 52 and the upper bearing portions 54.
Oil holes, not shown, are formed in at least one of the lower bearing portions 52 and the upper bearing portions 54 for supplying oil thereto, i.e., to the journal portions of the camshafts retained between the lower bearing portions 52 and the upper bearing portions 54. A second oil passage, also not shown, for supplying oil to these oil holes is formed in the connecting body 46, not the cylinder head 12.
The lash adjuster support portion 40 is provided extending in the lengthwise direction of the internal combustion engine 10. A plurality of support portions 56 for each cylinder are formed in the lash adjuster support portion 40. More specifically, an equal number of support portions 56 as there are intake and exhaust valves 20 and 22 are formed on both the intake side and the exhaust side (only the support portions 56 on the exhaust side are shown in the FIG. 2).
As shown in FIG. 1, the lash adjuster support portion 40 retains lash adjusters 60 in the support portions 56 on the intake side and the exhaust side. The lash adjusters 60 have rocker arm support portions 62 on their upper ends. These lash adjusters 60 receive a supply of hydraulic pressure with which they urge the support portions 62 upward with a predetermined force.
A first oil passage, not shown, for supplying that hydraulic pressure to the lash adjusters 60 is provided in the lash adjuster support portion 40. This first oil passage is formed extending in the lengthwise direction through the center of the lash adjuster support portion 40 so the distance to the lash adjusters 60 on the intake side is the same as the distance to the lash adjusters 60 on the exhaust side.
Rocker arms 64 on the intake side are placed on the lash adjusters 60 on the intake side. The other ends of the rocker arms 64 contact the upper ends of the intake valves 20. Also, the rocker arms 64 contact intake cams 66 positioned above them. That is, the rocker arms 64 on the intake side are sandwiched between the intake valves 20 and the lash adjusters 60 that are positioned below them, and the intake cams 66 that are positioned above them.
Similarly, rocker arms 68 on the exhaust side are placed on the exhaust valves 22 and the lash adjusters 60 on the exhaust side. The upper portions of the rocker arms 68 contact exhaust cams 70. That is, the rocker arms 68 on the exhaust side are sandwiched between the exhaust valves 22 and the lash adjusters 60 that are positioned below them, and the exhaust cams 70 that are positioned above them.
According to this structure, pressing force generated as the intake cams 66 rotate can be transmitted to the rocker arms 64 by the cam noses. The support portions 62 of the lash adjusters 60 are essentially not displaced so when the rocker arms 64 are pressed down from the downward pressing force, they pivot about the rocker arm support portions 62 and lift the intake valves 20 so that they open. As the cam noses pass through the point of contact with the rocker arms 64, the lift amount of the intake valves 20 decreases and the valves close again. Similarly, with this structure, as the exhaust cams 70 rotate, they cause the rocker arms 68 to pivot, thereby opening and closing the exhaust valves 22.
In the foregoing structure, the pressing force of the lash adjusters 60 is exerted as force that presses the rocker arms 64 or the rocker arms 68 against the intake cams 66 or the exhaust cams 70. Therefore, according to the structure of this example embodiment, the upper ends of the intake valves 20 or the exhaust valves 22, the rocker arms 64 or the rocker arms 68, and the intake cams 66 or the exhaust cams 70 are able to be stably maintained in a state of close contact over an extended period of time without affecting the wear of the intake cams 66 and the exhaust cams 70 or the rocker arms 64 and 68 and the like.
The intake valves 20 and the exhaust valves 22 operate appropriately in sync with the rotation of the intake cams 66 and the exhaust cams 70 as long as they are kept in the close contact state described above. Therefore, according to the structure of this example embodiment, the intake valves 20 and the exhaust valves 22 can be appropriately operated stably for an extended period of time regardless of wear of the rocker arms 64 and 68 and the like.
The camshaft structure of this example embodiment has a third oil passage, not shown, for spraying a shower of oil on the sliding surfaces of the intake cams 66 and the rocker arms 64, as well as on the sliding surfaces of the exhaust cams 70 and the rocker arms 68. This third oil passage is provided in the connecting body 46, not in the cylinder head 12.

Assembly Steps of the Support Structure According to the Example Embodiment

The camshaft support structure according to this example embodiment can easily be assembled by following the steps described below.
(Step 1) Fit the intake valves 20 and the exhaust valves 22, together with their associated elements (such as valve springs and the like), into the cylinder head 12. (Step 2) Fix the ladder frame type lower cam carrier 42 having the lash adjuster support portion 40 onto the cylinder head 12. (Step 3) Fit the lash adjusters 60 into the support portions 56 through the gaps in the ladder frame type lower cam carrier 42. (Step 4) Place the rocker arms 64 on the upper ends of the intake valves 20 and the lash adjusters 60 on the intake side. Similarly, place the rocker arms 68 on the upper ends of the exhaust valves 22 and the lash adjusters 60 on the exhaust side. (Step 5) Place the intake camshaft on the lower cam carriers 50 so that the rocker arms 64 contact the intake cams 66. Similarly, place the exhaust camshaft on the lower cam carriers 50 so that the rocker arms 68 contact the exhaust cams 70. (Step 6) Connect the upper cam carriers 44 to the lower cam carriers 50 so that the intake camshaft and the exhaust camshaft are properly retained.
After these steps have been performed, the head cover is then attached and the like, thus completing assembly of the internal combustion engine 10. In this way, the camshaft support structure of this example embodiment is able to easily be assembled in a production line without any trouble.

Advantages of the Support Structure According to the Example Embodiment

Simplification of the Cylinder Head

As described above, the camshaft support structure according to this example embodiment is such that the lash adjuster support portion 40 is provided separate from the cylinder head 12. Also, all of the necessary hydraulic pressure passages around the camshafts are formed not in the cylinder head 12 but in the connecting body 46 which is arranged above the cylinder head 12. Therefore, the camshaft support structure according to this example embodiment simplifies the structure of the cylinder head 12.
The lash adjuster support portion 40 can be manufactured by a die-cast manufacturing method. More specifically, a manufactured article in which the lash adjuster support portion 40 and the ladder frame type lower cam carrier 42 are integrally provided can be manufactured by a die-cast manufacturing method. Therefore, even if the lash adjuster support portion 40 is separate from the cylinder head 12, it will not adversely affect the productivity of the internal combustion engine 10 much.
On the other hand, the cylinder head 12 is manufactured by a kind of casting which is represented by gravity casting. When casting is used as the method of manufacture, productivity increases the simpler the structure of the article to be manufactured. In this example embodiment, because the structure of the cylinder head 12 has been simplified, the cylinder head 12 can be manufactured with high productivity. Also, because the productivity of the cylinder head 12 increases, the support structure according to this example embodiment increases the overall productivity of the internal combustion engine 10.
Also, simplifying the structure of the cylinder head 12 enables the unnecessarily thick portions included in the cylinder head 12 to be reduced, which in turn reduces the mass of the cylinder head 12 as well as makes it smaller. Therefore, the support structure according to this example embodiment promotes both weight and size reductions of the internal combustion engine 10.
Moreover, simplifying the structure of the cylinder head 12 consequently simplifies the die used to manufacture the cylinder head 12. The simpler the structure, the lower the cost of the die. Therefore, the support structure according to this example embodiment also promotes a cost reduction of the internal combustion engine 10.
Also, oil dripping down on the upper portion of the cylinder head 12 tends to circulate appropriately instead of accumulating in a specific place the simpler the structure of the cylinder head 12. As circulation of the oil improves, the bubble fraction of the oil decreases, there is less oil sludge, and less oil flows into the PCV (Positive Crankcase Ventilation). As a result, the support structure of this example embodiment realizes the following: higher speed/higher output of the internal combustion engine 10, improved reliability of the internal combustion engine 10, suppression of oil degradation and oil consumption, and cleaner exhaust gas, and the like.
(Making the Lash Adjuster Support Portion Out of Magnesium)
As described above, with this example embodiment, the lash adjuster support portion 40 and the ladder frame lower cam carrier 42 are integrally formed of magnesium. Magnesium is lighter than aluminum or cast iron of which the cylinder head 12 is made. Therefore, this structure enables the internal combustion engine 10 to be lighter and have a lower center of gravity at the same time.
(Increasing the Rigidity of the Cam Carrier)
With the support structure of this example embodiment, the plurality of lower cam carriers 50 arranged in between the cylinders are connected by the outer frame portion 48 as well as by the lash adjuster support portion 40, which increases the rigidity of the lower cam carriers 50. Because of this, this support structure of the example embodiment enables increases the rigidity of the lower cam carriers 50 to be increased while having them be made of magnesium.
(Advantages Related to the First Oil Passage)
As described above, with the support structure according to this example embodiment, the first oil passage is provided in the lash adjuster support portion 40 so that the distance to the lash adjusters 60 on the intake side and the distance to the lash adjusters 60 on the exhaust side are the same. This structure enables oil to be supplied to the lash adjusters 60 on both the intake side and the exhaust side always under the same conditions. Therefore, the support structure of this example embodiment can always get the best performance out of both lash adjusters 60.
(Combining Desired Oil Passages)
The support structure of this example embodiment requires that oil be supplied around the camshaft to the lash adjusters 60, the bearing portions, and the sliding surfaces of the cams and the rocker arms 64 and 68.
The first oil passage, the second oil passage, and the third oil passage which are necessary for supplying oil to these parts are formed in the connecting body 46 that includes the lash adjuster support portion 40, as described above, and are all connected to one another. Therefore, when oil is supplied to one of the passages, it can be supplied to all of the desired locations. Therefore, with the support structure of this example embodiment, the mechanism for supplying oil to the desired locations can be realized by an extremely simple structure. With respect to these points as well, the support structure of this example embodiment is able to improve productivity and reduce costs.
In the example embodiment described above, the lash adjuster support portion 40 is integrally formed with the lower cam carriers 50, but the invention is not limited to this structure. That is, the lash adjuster support portion 40 may also be formed as a member that is separate from the lower cam carriers 50, and then connected to the lower cam carriers 50 by fastening bolts or the like. In this case, the lower cam carriers 50 may be made of aluminum or cast iron, and only the lash adjuster support portion 40 made of magnesium.
Also, in the example embodiment described above, the lash adjuster support portion 40, the ladder frame type lower cam carriers 42, and the upper cam carriers 44 are made of magnesium, but they are not limited to being made of magnesium. That is, the lash adjuster support portion 40 and the like may also be made of resin composite which is lighter than aluminum or cast iron. Moreover, when reducing the weight and lowering the center of gravity of the internal combustion engine 10 are not extremely important, the lash adjuster support portion 40 and the like may be made of aluminum or cast iron.
Also, in the example embodiment described above, the plurality of lower cam carriers 50 are connected by the outer frame portion 48, but the invention is not limited to this structure. That is, each lower cam carrier 50 arranged between cylinders may be a separate member independent from the others, i.e., not connected by the outer frame portion 48.
Also, in the example embodiment described above, the upper cam carriers 44 are formed as independent members and are individually connected to the lower cam carriers 50, but the invention is not limited to this structure. That is, the upper cam carriers 44 may also be integrally formed with the head cover of the internal combustion engine 10.
Also, in the example embodiment described above, the lash adjuster support portion 40 is connected to the lower cam carriers 50, but the invention is not limited to this structure. That is, as long as the lash adjuster support portion 40 is provided separately from the cylinder head 12, it may ultimately be fixed to the cylinder head 12. In this case, however, it is necessary to provide an oil passage in the cylinder head 12 for supplying oil to the first oil passage in the lash adjuster support portion 40 which makes the structure of the cylinder head 12 slightly more complex than the structure of the example embodiment.
While the invention has been described with reference to exemplary embodiments thereof, it is to be understood that the invention is not limited to the exemplary embodiments or constructions. To the contrary, the invention is intended to cover various modifications and equivalent arrangements. In addition, while the various elements of the exemplary embodiments are shown in various combinations and configurations, which are exemplary, other combinations and configurations, including more, less or only a single element, are also within the spirit and scope of the invention.

Claims

1-12. (canceled)

13. A camshaft support structure of an internal combustion engine, comprising:

a cylinder head;

a camshaft;

a cam;

a rocker arm that abuts against the cam;

a lash adjuster that supports the rocker arm;

a lower cam carrier that supports the camshaft from the cylinder head side;

an upper cam carrier that supports the camshaft from the side opposite the lower cam carrier; and

a lash adjuster support portion which is provided separately from the cylinder head and connected to the lower cam carrier and supports the lash adjuster, wherein the lash adjuster support portion is not directly connected to the cylinder head.

14. The camshaft support structure of an internal combustion engine according to claim 13, wherein the internal combustion engine is provided with a plurality of the lower cam carriers, and the plurality of lower cam carriers are connected together by a frame to form a ladder frame type lower cam carrier.

15. The camshaft support structure of an internal combustion engine according to claim 13, wherein the lash adjuster support portion is integrally formed with the lower cam carrier.

16. The camshaft support structure of an internal combustion engine according to claim 13, wherein the lash adjuster support portion is made of material that is lighter than aluminum.

17. The camshaft support structure of an internal combustion engine according to claim 16, wherein the lash adjuster support portion is made of one of magnesium and resin composite.

18. The camshaft support structure of an internal combustion engine according to claim 13, wherein the internal combustion engine has a plurality of cylinders arranged inline, a plurality of the lash adjusters are provided corresponding to the plurality of cylinders, and the lash adjuster support portion extends so as to support all of the lash adjusters and is provided with a first oil passage for supplying oil to the lash adjusters.

19. The camshaft support structure of an internal combustion engine according to claim 18, wherein the internal combustion engine is provided with an intake valve and an exhaust valve for each cylinder, and the lash adjuster support portion is arranged in between the intake valve and the exhaust valve.

20. The camshaft support structure of an internal combustion engine according to claim 19, wherein the first oil passage is formed such that the distance from a predetermined position in the first oil passage to the lash adjuster on the intake side of a given cylinder is the same as the distance from the predetermined position in the first oil passage to the lash adjuster on the exhaust side of the given cylinder.

21. The camshaft support structure of an internal combustion engine according to claim 18, further comprising:

a second oil passage for supplying oil to a bearing portion formed by the lower cam carrier and the upper cam carrier,

wherein the second oil passage is formed in a connecting body that includes the lash adjuster support portion, the lower cam carrier, and the upper cam carrier, and communicated with the first oil passage.

22. The camshaft support structure of an internal combustion engine according to claim 18, further comprising:

a third oil passage for supplying oil to sliding surfaces of the cam and the rocker arm,

wherein the third oil passage is formed in a connecting body that includes the lash adjuster support portion, the lower cam carrier, and the upper cam carrier, and communicated with the first oil passage.

23. An assembly method of a camshaft support structure of an internal combustion engine, comprising the steps of:

fixing a lower cam carrier having a lash adjuster support portion onto a cylinder head into which an intake valve and an exhaust valve have been fitted;

setting a lash adjuster in the lash adjuster support portion;

placing a rocker arm on the intake side and a rocker arm on the exhaust side;

placing camshafts on the lower cam carrier so that the rocker arm on the intake side contacts an intake cam and the rocker arm on the exhaust side contacts an exhaust cam; and

assembling an upper cam carrier to the lower cam carrier.